1. Field of the Invention
The present invention relates generally to the detection of species ionized by radiant energy. Particularly, the present invention relates to a method of detecting hydrides or mercury ionized by radiant energy using photoionization.
2. Description of the Prior Art
Photoionization as a scientific concept has been known for some time. The first application of photoionization detection was as a gas chromatography (GC) ion detector for hydrocarbons. In a photoionization detector high-energy photons, typically in the ultraviolet (far UV) range, break molecules into positively charged ions. As compounds elute from the GC's column they are bombarded by high-energy photons and are ionized when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.
More particularly, the photoionization process is initiated when a photon of sufficient energy (from a short wavelength UV lamp) is absorbed by a molecule. This results in the creation of a positive ion and an electron as shown below:R+hνR++e−where:                R=an ionizable species        hν=a photon with sufficient energy to ionize species R        
In the ion chamber, the ions (R+) formed by absorption of the UV photons are collected by applying a positive potential (100-200 V) to the accelerating electrode and measuring the current at the collection electrode. The current produced is proportional to the concentration over a very wide range. Once the collection electrode is shielded from the UV to reduce the background current, and the geometry is axial, the field strength is given by:E=V/(2.3r Log a/b)where                V is the applied voltage between the collector of radius a and the accelerating electrode of radius b, and        E is the electric field at any point in distance r from the center of the accelerating electrode.        
As a result, the field increases rapidly as r→b. The most effective ion chamber design is where there is a coaxial configuration. These design characteristics provide a PID with the lowest background, best possible sensitivity and widest dynamic range.
Hydride generation is a procedure commonly used for sensitivity enhancement in a variety of instrumental methods for measuring trace levels of As, Se, Sb, Sn, Ge, Te, and Bi (and sometimes Pb) in aqueous solutions and wet-ashed solid samples. A hydride is a compound in which one or more hydrogen atoms have reducing, or basic properties. In hydrides, hydrogen is bonded to a more electropositive element such as a metal. When metal hydride forming compounds in solution are treated with a reducing agent, the hydride MH3 (g) is formed.
For atomic spectral analysis, the hydride technique typically results in several orders of magnitude improvement in concentration sensitivity over conventional nebulizer sample introduction. The two instrumental methods most commonly coupled to hydride preconcentration are atomic absorption and plasma emission spectrometry. When either of these two detectors are used with the most recent commercially available, state-of-the-art, automated, continuous-flow hydride generators, solution-phase concentration detection limits in the range of 0.2-0.4 ppb As, Se, and Sn can be routinely achieved. Generally liquid nitrogen was used to preconcentrate the sample.
The photoionization approach to hydride detection replaces atomic absorption and plasma emission detectors. A photoionization detector (PID) and a liquid nitrogen cold trap (i.e. a concentrator) provides several orders of magnitude of sensitivity improvement over the best existing continuous-flow hydride generation atomic absorption and plasma emission systems for As, Se, and Sn determination (without suffering any significant selenium hydride loss).